Field of the Invention
The present invention concerns magnetic resonance imaging, and in particular concerns magnetic resonance imaging techniques for multi-slice imaging.
Description of the Prior Art
Many magnetic resonance imaging (MRI) examinations that are currently conducted incorporate two-dimensional (2D) measurements. In such conventional measurements, magnetic resonance data are acquired from a stack of parallel, adjacent 2D slices, with the stack covering the entirety of the region of interest in the body of the patient. Usually, nuclear spins in each slice are excited separately, i.e., the excitation takes place slice-by-slice, and the resulting magnetic resonance signals are subsequently also acquired separately (slice-by-slice). This means that the data acquisition for one slice is independent of the data acquisition from the other slices in the stack. Therefore, the number of slices in the stack can be selected without limitation, because each slice forms its own individual part of the overall measurement.
A more recent technique is known as simultaneous multi-slice (SMS) imaging, which can accelerate scans by exciting nuclear spins in, and acquiring resulting magnetic resonance signals from, multiple slices at the same time. In order to do so, SMS imaging employs radio-frequency (RF) excitation pulses that differ in design from RF excitation pulses that are used in non-SMS imaging. The RF excitation pulses that are used in SMS imaging are called multiband pulses, and are designed so as to excite two or more (usually equidistant) slices simultaneously. The number of slices that are excited at the same time (bands) is called the SMS-factor.
One consequence of SMS imaging is that the number of slices that are acquired for a given volume of the patient can only be an integer multiple of the SMS-factor. The number of multiband RF excitation pulses that are used for the acquisition of the entire stack of slices must be an integer number. Consequently, the number of scanned slices is the product of the number of such RF pulses and the SMS-factor. This limitation leads to the situation that, for example, an SMS-factor of two allows only even numbers of slices to be scanned, and an SMS-factor of five, for example, only allows 5, 10, 15, 20 . . . slices to be scanned.
Many imaging situations, however, require that the number of slices be selected without restriction. Such is the case, for example, in neuro-imaging, wherein the slice numbers are often odd, and hence are not compatible with even SMS-factors. Moreover, computer program tools are commercially available that automatically determine and adapt the number of slices from which MR data are to be acquired to the dimensions of the head of a patient. Since this number is patient-specific, feasible acceleration factors which are a divisor of the number of slices are also be patient specific. It may possibly be odd or even and, if the adapted number of slices turns out to be odd, conventional SMS imaging with an even acceleration factor is then not available for conducting the examination.
Heretofore, it has been accepted as a technical limitation of SMS imaging that the selection of the number of slices will be subject to the aforementioned limitations. In addition to then making SMS imaging unavailable for certain examinations, this also leads to the inconvenience that images acquired by SMS imaging cannot always match the slice numbers and positions of images acquired by non-SMS imaging. If 25 slices have been scanned, for example, with a TSE (turbo spin echo) sequence without SMS imaging, and 26 slices have been scanned, for example, with a diffusion EPI (echo planar imaging) sequence with SMS imaging covering the same volume, the respective images acquired with the different sequences cannot be viewed side-by-side.